According to University College London (UCL), a change in sense of humour can provide a "red flag" to family members and doctors, giving an early warning sign that neurodegenerative disease has begun.

Many family members notice behavioural and personality changes in their loved ones in the years before dementia is diagnosed, and researchers speculated that humour may be one of the most obvious alterations.

Friends or relatives of 48 people with different forms of fronto-temporal dementia and Alzheimer's disease were asked to rate their preference for different kinds of comedy and how it had changed over 15 years. The researchers found that people with both types of dementia preferred slapstick humour to more cerebral satirical comedy, such as 'Yes, Minister', or absurdist comedy, such as that of Monty Python and The Goons, even if they had previously been fans of more complex comedy.

People with fronto-temporal dementia also appeared to develop a darker sense of humour. And their jokes tended to be graphic, smutty or childish in their subject, according to the study. Many stopped laughing altogether. The same was not true of Alzheimer's sufferers.

While Alzheimer's disease is the leading cause of dementia, fronto-temporal dementia is the most common cause of dementia in the under-55s.

Friends and relatives reported seeing the changes in sense of humour for both forms of dementia on average at least nine years before the start of more typical dementia symptoms such as memory loss.